Cargo transported overland is often carried in a standard ISO-type cargo container which is secured to a trailer which, in turn, is connected to and drawn by a tractor. The two standard size cargo containers are 20 feet and 40 feet in length. A fully loaded 20 foot cargo container can typically weigh 44,800 pounds. In the early 1980's, a new heavier capacity cargo container was introduced worldwide. This new 20 foot container can weigh up to 52,920 pounds fully loaded.
The load capacity of the tractor-trailer is limited worldwide primarily by the deemed effect of the weight on the road. Factors which are considered include the gross weight and the weight on each axle of the trailer. In recent years, the trucking industry in Canada and the U.S.A. has been pressing for size and weight increases in order to increase carrying capacity and thus their productivity. The industry argues that much greater pay-loads are possible for the same hauling costs by reducing the number of trips required. It is argued that this also will increase safety by reducing the exposure. Opponents contend that increasing the permissible size and weight will cause more rapid deterioration of existing roads and bridges and require greater public expenditures for new roads, bridges, and exit ramps.
In Canada, regulations governing the size and weight of commercial vehicles are the responsibility of individual provinces and territories. Ontario currently limits the permissible weight on single axles to a maximum of 10,000 kilograms and 19,100 kilograms for tandem axles. The maximum gross weight allowed is 140,000 pounds or 63,500 kilograms which is achievable with a tractor plus two semi-trailers with a total of seven or eight axles. In Quebec, tandem axles may be loaded to 20,000 kilograms. The Canadian limits are generally greater than those permitted in the United States.
In the United States, the laws are generally the responsibility of the individual states, except on the U.S. Interstate Highway System which is under federal control. For example, the new 1983 legislation requires the states to permit up to 20,000 pounds single, 34,000 pounds tandem and 80,000 pounds gross vehicle weight on the Interstate Highways.
The trucking industry is further regulated in the United States by the Interstate Bridge Laws which dictate the maximum load which can be transported over bridges. The Federal Bridge Formula, used to calculate this limit, is a function of the number of axles and the distance between extremes of any group of two or more consecutive axles. Accordingly, the longer the trailer, the less stress there is on the bridge. While the load capacity can be increased by extending the wheelbase, the cargo container has to be centrally mounted to ensure proper weight distribution to the axles. However, central mounting of the cargo container on the trailer, for example by mounting a 20 foot container on a 40 foot chassis, poses problems in the loading and unloading of the cargo container, as will be discussed in more detail hereinafter.
The standard ISO-type cargo containers are typically mounted on trailers which are drawn by tractors. It is not uncommon to see a second trailer connected to the rear of the first trailer.
There are two methods of connecting trailers referred to by those skilled in the art as "A trains" and "B trains". In the A train configuration, a trailer-converter dolly is connected to the rear of the first trailer and the second trailer is then connected to the rear of the dolly. There are three articulation points in an A train: the first is a fifth wheel connection between the first trailer and the tractor, the second is a pintie hook between the dolly and the rear of the first trailer, and the third is a fifth wheel connection between the second trailer and the dolly. The A train configuration has the advantage that rear end loading and unloading of the cargo container is facilitated by disconnection of the dolly so that the rear of the first trailer can be butted against the loading dock. However, the double articulation at the dolly may be dangerous because it permits buckling and similar motions between the first and second trailers.
In the B train configuration, the second trailer overlaps and is partially supported by the rear axles of the first trailer. The B train has only two articulation points: the first at the fifth wheel connection between the first trailer and the tractor and the second at a second fifth wheel connection between the second trailer and the first trailer. This combination is generally more stable than the A train configuration because of the fewer articulation points. Moreover, the second fifth wheel connection has more structural integrity and better handling characteristics than the pintie hook connection at the first trailer-converter dolly. As such, the B train configuration is becoming the configuration of choice in the transport of two cargo containers. The disadvantage of the B train configuration is that rear end loading of the cargo container at a loading dock is difficult because of the substantial gap between the rear of the cargo container and the loading dock.
During loading and unloading of a cargo container, the trailer is usually backed up to a loading dock and the cargo is moved into or out of the container frequently using forklift trucks.
Typically the lead trailer of a B train tractor-trailer combination has an upper flatbed portion at the forward portion of the trailer and a lower deck portion which is located at the rear of the trailer. This step from the flatbed to the deck causes problems in the loading and unloading of cargo from the container. The problem is further compounded by the fifth wheel mounted on the deck of the lead trailer.
Arguin (U.S. Pat. Nos. 4,400,004, 4,526,395, 4,598,924, and 4,673,191) teaches one solution to this problem by the provision of a platform which is hinged to the chassis of the trailer at the rear end of the cargo container. The platform normally lies against the container door when the container is being transported. When the container is to be loaded or unloaded the platform is lowered to a horizontal position. A plate attached to the platform lies on the fifth wheel and legs are provided to give additional support for the platform. This allows a forklift truck to enter the container from the loading dock. Although the floor of the cargo container is at the same level as the loading dock, there is still a gap between the container and the loading dock. This is particularly a problem in inclement weather and in the transport of frozen foods. Many loading docks are provided with a seal against which the rear of the container is pressed to ensure that loading and unloading is done without loss of either cooled or heated warehouse environment.
U.S. Pat. No. 5,026,228 (Mansfield) discloses a means for moving a cargo container attached to a horizontal bar on the deck portion at the rear of the trailer. The bar, and therefore the cargo container, is moved by a hydraulic actuator to the rear of the trailer so that there is no gap between the container and the loading dock. The deck has a pair of strips of a low-friction material such as molyfilled nylon to allow the horizontal bar to move more easily across the deck. Upwardly protruding regions of the beams on the flatbed portion of the trailer are also provided with strips of low-friction material to allow the container to slide back and forth on the flatbed. The cargo container has slots which engage these upwardly protruding regions of the beams to prevent sideways movement of the container as the container is moved. Although the possibility of a lockdown mechanism at the front of the container to prevent movement of the container during transport is alluded to, it will be understood by those skilled in the art that alignment of the container, especially when fully loaded, during movement to the docking position and locking of the cargo container to prevent any movement whether sideways or vertically are major considerations for a workable solution. Furthermore, during transport the strips of low-friction material on the deck and the upwardly protruding regions of the beams will become soiled with dust, mud, gravel, snow, ice, slush, etc. that will prohibit movement of the cargo container on the strips.
It is advantageous to have the cargo container butt against the edge of the loading dock so that forklift trucks can be driven directly into the cargo container from the loading dock without the need for a gap bridging platform or a ramp. For example, in the transport of frozen foods it is desirable and may even be mandatory to provide a seal in the space between the rear of the cargo container and the entryway of the loading dock through which the frozen cargo is loaded and unloaded in order to prevent exposure of the cargo to temperatures above freezing. Moreover, it is often desirable to have little or no gap between the cargo container and the loading dock to protect forklift operators and cargo from inclement weather. A typical loading dock is about 50 to 53 inches high although some have means for adjustment from 48 to 54 inches. It can be appreciated by those skilled in the art that it is also desirable to have the floor of the cargo container at the same height as the floor of the loading dock.